This invention relates generally to the field of solar power generation and, more particularly, to methods and systems to allow for a high DC source voltage in a solar power inverter system.
Solar power generation is becoming a progressively larger source of energy throughout the world. Solar power generation systems typically include one or more photovoltaic arrays (PV arrays) having multiple interconnected solar cells that convert solar energy into DC power through the photovoltaic effect. In order to interface the output of the PV arrays to a utility grid, a power converter system is used to change the DC current and DC voltage output of the PV array into a 60/50 Hz AC current waveform that feeds power to the utility grid.
Various power converter systems exist for interfacing the DC output of a PV array (or other DC power source) with the AC grid. One implementation of a power converter system includes two stages, a boost converter stage and an inverter stage. The boost converter stage controls the flow of DC power from the PV array to a DC bus or DC link (hereinafter referred to as the “DC link”). The inverter stage converts the power supplied to the DC link into a suitable AC waveform that can be output to the AC grid.
Situations arise in which it is necessary to accommodate a PV array (or other DC power source) that has a high open-circuit voltage, such as an open-circuit voltage that is very close to the blocking voltage rating of the semiconductor devices employed in the power inverter system. E.g. array open circuit voltage is 1000 Vdc and the blocking voltage of the semiconductor devices is 1200 Vdc. Power inverter semiconductor devices, such as insulated gate bipolar transistors (IGBTs), are typically selected to accommodate the maximum power voltage of the PV array, not the open-circuit voltage of the PV array. The limiting factor in starting up at high voltage is the voltage overshoot at turn-off of the IGBTs.
In view of the foregoing, there is a need for a solar power inverter system and method of operation that allows for a high DC source voltage during start-up conditions. The inverter system should prevent the DC link voltage from reaching or exceeding the inverter system semiconductor device blocking voltage(s) during PV inverter start-up.